Tetrachloroiodic acid
Tetrachloroiodic acid is an inorganic compound with the formula HICl4, which acts the only example of a stable acid derived from a polyhalide. An orange crystalline tetrahydrate is known. No anhydrous tetrachloroiodic acid is known to exist; rather, it is isolated as hydronium salts of the tetrachloroiodate anion. It is unstable in air.
Synthesis
Tetrachloroiodic acid may be formed by dissolution of iodine trichloride in concentrated hydrochloric acid:Tetrachloroiodic acid may also be made by adding hydrochloric acid to iodates or periodates, or by passing chlorine through a solution of iodine in concentrated hydrochloric acid,
although it quickly decomposes into iodine and iodate when introduced into less acidic conditions.
Physical properties
Tetrachloroiodic acid forms a crystal hydrate which has orange crystals that are unstable in air and melt by dissolving in their own water of crystallization at 19 °C. Crystals of HICl4·4H2O contain square planar tetrachloroiodate anions associated with various hydronium cations such as H5O2+, showing structural similarities to the hydrates of other chloride-based inorganic acids, most notably HAuCl4·4H2O.Hydrates of the sodium and potassium salts of the tetrachloroiodate anion have been isolated, along with tetrachloroiodates of various alkaloids.